The metal working industry utilizes various coolants and lubricants during cutting and forming operations. These fluids range from pure oils, oil and water mixtures, and non-oils synthetics. Because of the forcefulness with which these fluids are applied during metal working operations and the heat generated where the cutting tool meets the workpiece, a fine mist or aerosol in the 0 to 2 micron range is generated. To arrest this mist, a high efficiency filter is required.
If the machine tool is free standing, the aerosol mist can be pulled away from the area using a hood over the cutting area. Alternatively, newer machine tools now employ enclosures around the cutting area, which allow the mist to be withdrawn from a single location.
Filters that have traditionally been used for mist collection originated in the heating, ventilation, and air conditioning (HVAC) industry. These filters generally incorporate a fiberglass mat with a synthetic non-woven layer on the backside for additional strength. This material can be fabricated in a variety of styles such as folded pockets, cassettes, etc., typically for use in a filtration cabinet.
A system widely in use for aerosol mist collection employs relatively thick filter bags which are folded into large pleats arranged with respect to the airflow, also known as pocket style HVAC filters. These pocket filters are typically not constructed with a separation structure, allowing the layers collapse under pressure.
The oil in the mist is absorbed by the relatively thick, porous pockets causing the bags to gradually become saturated with oil. The bags eventually collapse under the weight of the oil, causing the air to be forced through a smaller surface area. The reduced surface area of the filter increases air velocity through the filter, resulting in some of the oil being forced through the bags, contaminating the downstream air.
Consequently, these filters have a generally declining efficiency as the oil collects in the filter. As a result of the declining efficiency, the pressure drop across the filter does not increase, making it difficult to determine when the filter needs to be replaced. Disposal of such filters is also difficult because of environmental concerns over their oil content.